The present invention relates to an apparatus or circuit for reducing a ghost phenomenon in a television receiver and in particular relates to an analog-to-digital conversion circuit suited for use in the ghost reduction circuit.
When an electromagnetic wave signal (desired wave) incoming directly from an antenna for transmission is received by a receiver antenna simultaneously with a reflected signal travelling over a longer path due to reflection at a large building, hill or the like, there makes appearance an undesired duplicate image deviated from the desired image on the screen of the television receiver. In other words, the so-called "ghost" is generated. In the television receiver, the phenomenon of ghost is one of the major causes for degradation in the quality of image. Accordingly, various measures have heretofore been taken for reducing or preventing the ghost. As one of the approaches, there can be mentioned a ghost reducing system in which a transversal filter is made use of for the video signal. In this ghost reducing system, a plurality of delay elements each exhibiting a minute delay time determined in dependence on the highest frequency component contained in the video signal are connected in series to one another, wherein output signals from the delay elements are appropriately weighted and added together in a factor applying circuit, to derive a compensated signal from which the ghost component is substantially eliminated.
Various circuit configurations for the ghost reduction circuit in which the transversal filter is used have heretofore been proposed. An example of such circuits is shown in a block diagram in FIG. 1 of the accompanying drawings. In this figure, a reference numeral 1 denotes an input terminal for a video signal, 2 denotes an output terminal for the video signal, 3 denotes a transversal filter, 4 denotes a subtraction circuit or subtracter, 5 denotes a reference signal generator circuit, 6 denotes a differentiating circuit or differentiator, 7 denotes a comparator, 8 denotes a shift register formed of 8 bit shift registers SN74164 of Texas Instruments, 9 denotes another subtractor, 10 denotes a tap gain memory, 11 denotes a digital-to-analog (D/A) converter, 12 denotes a synchronizing signal separator, and 13 denotes a timing generator.
FIG. 2 is a block diagram illustrating in detail an arrangement of the transversal filter 3 shown in FIG. 1. Referring to FIG. 2, a reference numeral 14 denotes a summing circuit or an adder, 15.sub.1 to 15.sub.n denote delay elements each having a delay time .tau. where n may be 100, by way of example, and 16C.sub.1 to 16C.sub.n denote tap amplifiers, respectively. With the term "tap amplifier", it is intended to mean the amplifier whose amplification gain can be varied in accordance with a control voltage supplied thereto from the tap gain memory 10 through the D/A converter 11.
In the first place, the operation of the circuit shown in FIG. 1 will be briefed.
The video signal applied to the input terminal 1 is transferred through the transversal filter 3 to the output terminal 2 to be supplied to a succeeding circuit stage. In case the input video signal contains the ghost component, the latter is substantially eliminated or reduced before being sent out to the output terminal 2. To this end, it is necessary to detect the ghost component possibly contained in the video signal produced by the transversal filter 3.
In this connection, it will be technically convenient for the ghost detection to select the vertical synchronizing signal and detect the ghost component possibly superposed on the vertical synchronizing signal. The reason may be explained as follows. Suppose that the ghost component superposed on the picture signal is detected. Then, difficulty will be encountered in the detection of the ghost, because the picture signal varies constantly. Accordingly, it is preferred to detect the ghost component superposed on the vertical synchronizing signal.
The vertical synchronizing signal is separated from the video signal applied to the input terminal 1 by means of the synchronizing signal separating circuit 12. The synchronizing signal thus separated is supplied to the timing generator circuit 13 to be made use of as the reference signal for generating a timing signal. The reference signal generator circuit 5 which may be constituted by a one-shot multivibrator, by way of example, generates the reference signal, i.e. the ghost-free sync reference signal on the basis of the vertical synchronizing signal under the timing commanded by the timing generator circuit 13. Accordingly, the ghost component superposed on the vertical synchronizing signal of the video signal can be determined by subtracting through the subtracter 4 the vertical synchronizing signal contained in the video signal outputted from the transversal filter and the vertical synchronizing signal outputted as the reference signal by the reference signal generator 5 from each other.
The ghost component is applied to the differentiating circuit 6 whose output signal is a positive going pulse (.phi.=0) for the in-phase ghost while the differentiated output signal is a negative going pulse (.phi.=180.degree.) for the out-of-phase ghost. The output signal of the differentiating circuit 6 is digitalized (binary state-encoded with respect to ghost component polarity) through the comparator 7, the resulting digital output being loaded into (written in) the shift register 8. The timing for the loading operation is controlled by the timing generator 13. The data of gain stored in the tap gain memory 10 is updated in dependence on the data read out from the shift register 8. More specifically, data read out from the memory 10 is correctively modified through the subtracter 9 in accordance with the data read out from the shift register 8, wherein the corrected (updated) data is again written in the memory 10.
Upon completion of the process described above, the tap gain data is subsequently read out from the memory 10 to be supplied to the D/A converter 11, the analog output signal of which is then applied to the amplifiers 16C.sub.1 -16C.sub.n (FIG. 2) constituting parts of the transversal filter 3 for controlling the amplification factor or gain thereof. This results in that the video signal which has the ghost component reduced is outputted from the transversal filter 3. By repeating the process described above for 8 seconds, for example, there can be ultimately obtained the video output signal from the transversal filter 3 which signal has substantially no ghost component superposed thereon.
In the foregoing, the outline of the operation of the ghost reduction circuit shown in FIG. 1 has been described. Now, the ghost detection signal processing operation will be described below in detail by referring to FIG. 3 which illustrates signal waveforms produced at main circuit points in the circuit shown in FIG. 1.
Referring to FIG. 3, a waveform (a) represents the vertical synchronizing signal outputted as the reference signal from the reference signal generating circuit 5. The waveform (a) has a leading edge indicated by F. The vertical synchronizing signal contained in the video signal outputted from the transversal filter 3 is represented by a waveform (b), wherein the superposed ghost component is represented by a hatched portion. A waveform (c) represents the ghost component resulting from the subtraction performed by the subtracter 4, while a waveform (d) represents the differentiated output pulse P resulted from differentiation of the ghost signal (c). When a control signal (gate pulse) is supplied to the shift register 8 from the timing generator circuit 13 in timing with the leading edge F of the vertical synchronizing signal to thereby initiate the operation of the shift register 8 at that time point, the binary output (indicating the ghost polarity) of the pulse signal P is fetched by the shift register 8 at a time point delayed by T relative to the appearance of the leading edge F for the purpose of performing the low-speed data processing to the memory 10 mentioned hereinbefore. As the consequence, ghost data or information including a series of bits is stored in the shift register 8 and sequentially outputted to the subtracter 9.
Subsequently, the correcting or updating operation of data stored in the tap gain memory 10 is started. The tap gain memory 10 has addresses which correspond to the tap amplifiers 16C.sub.1, 16C.sub.2, . . . and 16C.sub.n (shown in FIG. 2), respectively, whereby the tap gain data placed at the respective addresses are correctively modified in the order of the input signals having the minimum delay time to the input signal of the maximum delay time, i.e. in the order of the input signals corresponding to the amplifiers 16C.sub.1, 16C.sub.2, 16C.sub.3, . . . and 16C.sub.n.
Upon completion of correction of the data stored in the tap gain memory, operation is then triggered to supply the tap gain data thus updated to the individual tap amplifiers 16C.sub.1, 16C.sub.2, . . . and 16C.sub.n, respectively, which are included in the transversal filter 3. More specifically, data read out from the tap gain memory 10 are converted into the corresponding analog voltages, respectively, through the D/A converter 11 to be applied to the individual tap amplifiers 16C.sub.1, 16C.sub.2, . . . 16C.sub.n, respectively. The applied voltages are charged in capacitors of a small capacity (not shown) provided in association with the individual amplifiers 16C, respectively, to be thereby sampled and held. When the voltages representative of the tap gain data read out from the memory 10 have been completely applied to the corresponding tap amplifiers, respectively, the voltage application is again started, beginning with the tap amplifier 16C.sub.1. By repeating this process, the capacitors mentioned above are prevented from discharging.
The process including the detection of the ghost, the correction of data stored in the tap gain memory 10 and application of the control voltages to the associated tap amplifiers 16C as described above is performed once for one field of the video signal, since the vertical sync signal is made use of as the reference signal. This process is repeated until the ghost is no more detected. In this way, the ghost component can be progressively reduced.
In the ghost reduction circuit of the type described above, there arises a serious problem in conjunction with differentiation of the detected ghost component and the binary encoding of the differentiated signal as performed by the differentiating circuit 6 and the comparator 7 shown in FIG. 1. This problem will be discussed below in more concrete by referring to FIG. 4.
It is assumed that the output of the differentiating circuit (the circuit 6 shown in FIG. 1) is of such a waveform as illustrated in FIG. 4 at (a), wherein a broken line Th represents a threshold level of the succeeding comparator 7. In most of applications, the output signal of the differentiating circuit 6 has a DC level D which usually differs from the threshold level Th, as a result of which a DC offset .DELTA.D is involved between the DC level D and the threshold level Th, the DC offset .DELTA.D being difficult to be suppressed to or below 1 mV. Consequently, the output signal of the comparator 7 is of such a waveform as illustrated at (b) in FIG. 4. More specifically, the two-type output signal of the comparator 7 takes a value of "+1" for the portion exceeding the threshold value Th while taking a value of "-1" for the portion below the threshold level Th. The comparator output becomes "0" in absence of the ghost. Accordingly, the directions of corrections of data C.sub.N, C.sub.N+1, C.sub.N+2, . . . C.sub.N+n stored in the tap gain memory 10 (FIG. 1) are all positive because the binary output of the comparator 7 is at the level of "-1", except for the data corresponding to the differentiated pulse signal produced due to the ghost, as is shown at (c) in FIG. 4. Thus, the output signals of the individual tap amplifiers are of such waveforms as shown at (d) in FIG. 4, respectively, whereby an output signal of the transversal filter 3 representative of a sum of the output signals of these tap amplifiers is of such a waveform as shown at (e) in FIG. 4. As will be seen, the tap gain corresponding to the differentiated pulse produced due to the ghost component is decreased to be effective for cancelling the ghost components. In contrast thereto, all the other tap gains tend to be uniformly increased in an effort to cancel out the binary output signal of "-1" produced due to the presence of the DC offset .DELTA.D. In other words, the stepwise waveform is tilted.
Magnitude of the tilt is given by N.multidot..DELTA.D where .DELTA.D represents the DC offset and N represents the number of the tap amplifiers of the transversal filter. It is assumed by way of example, that the DC offset is sufficiently small so that .DELTA.D=1 mV, and that the delay time imparted to each delay element 15 is 100 nS. In order to be able to reduce the ghost of 10 .mu.S. N must be so selected that ##EQU1## Accordingly, magnitude of the tilt is given by EQU N.multidot..DELTA.D=100.times.1 mV=100 mV
In this connection, it is noted that the vertical sync signal has an amplitude of about 300 mV. This means that magnitude of the tilt amounts to one third of the amplitude of the vertical sync signal even when the DC offset is suppressed to 1 mV, to bring about remarkable degradation in the picture quality.
As a method of suppressing the tilt, there may be mentioned proposals disclosed, for example, in Japanese Laid-Open Patent Application No. 109023/1980 (Japanese Patent Application No. 15646/1979) and Japanese Laid-Open Patent Application No. 29552/1979.
The system disclosed in Japanese Laid-Open Patent Application No. 109023/1980 (refer to FIG. 4 of this prior application) is shown in FIG. 5 of the accompanying drawings. In this figure, I.sub.1 denotes an inverter amplifier, I.sub.2 denotes an inverter, S.sub.1 and S.sub.2 denote switches interlocked so that they are changed over every field, 19 denotes a capacitor, 20 denotes a resistor, and 21 denotes a comparator.
At the N-th field, the interlocked switches S.sub.1 and S.sub.2 are thrown to the respective upper contacts as viewed in FIG. 5. At the (N+1)-th field, these switches are changed over to the respective lower contacts.
FIGS. 6a to 6b illustrate waveforms of signals appearing, respectively, at circuit points (a) to (d) in the system shown in FIG. 5.
Description will be made of the assumption that the output signal of the differentiating Circuit 6 is of positive (+) polarity and that the input DC voltage to the comparator 21 is lower than the threshold level Th thereof.
The input signal to the comparator 21 at the N-th field is illustrated in FIG. 6a. Accordingly, a data signal of "+1" is supplied to the shift register 8 (FIG. 1) only when the differentiated signal is present, while otherwise a data signal of "-1" is supplied to the shift register 8, as is shown in FIG. 6b. On the other hand, at the (N+1)-th field, the signal input to the comparator 21 is lower than the threshold value Th, resulting in that the output signal of the comparator 21 is at "-1", as is seen from FIG. 6b, and that the output signal of the inverter I.sub.2 is "+1", as is shown in FIG. 6c. The data stored in the tap gain memory 10 (FIG. 1) are corrected on the basis of the binary (two-type) data thus obtained. This correction is performed once for every detection of ghost (1/60 sec..times.2). As a result, the DC offset of the comparator is compensated by twice ghost detections. The corrected values for the two successive fields are added together (refer to FIG. 6d). Accordingly, the corrected and summed value takes "+2" only for the differentiated signal produced in correspondence with the ghost while otherwise taking "0" notwithstanding of the DC offset. In the latter case, correction of the contents of the tap gain memory is not performed in appearance. Same applies true to the case where the output signal of the differentiating circuit is of inverse polarity or the case where the input DC level to the comparator 21 is higher than the threshold level.
As will be appreciated from the foregoing, the arrangement shown in FIG. 5 does not perform the correction of memory data in appearance even when an error due to the DC offset is present, whereby the occurrence of the tilt mentioned above in conjunction with the waveform shown at (e) in FIG. 4 can be prevented.
However, when consideration is made to the single field, the tilt mentioned hereinbefore which is reciprocated at a high speed is really produced. As the consequence, flicker of 30 Hz due to the tilt makes appearance on the image screen and provides an eyesore, giving rise to a new problem.
The system (analog-to-digital converter) taught in Japanese Laid-Open Patent Application No. 29552/1979 is designed to operate on substantially the same principle as mentioned above. More specifically, as an attempt to prevent the occurrence of error due to the analog-to-digital conversion, the converted value of an input signal per se which contains the error due to the conversion and the converted value containing the error due to conversion of the other input signal having the inverted polarity are subjected to addition/subtraction on the time series basis for obtaining the correctly converted value. In the ghost reduction circuit to which this prior art is applied, it requires two fields to obtain the correctly converted value because the input signals are present only in a predetermined time interval due to the fact that the vertical sync signal is utilized as the reference signal, presenting also a problem that the flicker is produced.